1. Field of the Invention
The present invention relates to ionizing radiation shields. More particularly, the present invention relates to a shielding composition for attenuating gamma rays and absorbing neutrons.
2. Discussion of Background
In working with high-level radioactive materials, such as spent nuclear fuels, nuclear waste and industrial radiation sources, the use of thick shielding, remote manipulation, or both is necessary to minimize radiation exposure to human operators.
Lead has often been used for gamma ray shielding because it is dense, easily worked and relatively inexpensive. Also, a lead shield can often be smaller than a comparable radiation shield made of virtually any other material so it takes up less space and is more portable.
However, lead is a toxic metal that is slowly attacked and corroded by air, water and soil acids. Also, water-soluble lead compounds, such as lead carbonate, tend to persist in the environment for long periods of time and are highly toxic to humans and other forms of life.
Lead tends to accumulate in the body, similar to other heavy-metal poisons, and continues producing toxic effects for many years after exposure. Therefore, it is desirable to eliminate lead from many of its present uses, including radiation shielding, and to find substitutes for lead.
Depleted uranium (chiefly uranium-238) is well known for use in absorbing gamma radiation. For example, Takeshima et al, in U.S. Pat. No. 5,015,863, discloses the use of depleted uranium particles for radiation shielding. Also, Barnhart et al, in U.S. Pat. No. 4,868,400, discloses the use of depleted uranium rods or small balls as radiation shielding in an iron cask for shipping and storing spent nuclear fuel.
However, U-238 is radioactive, with a half-life of about 4.5 billion years, and undergoes about 12,000 disintegrations per gram per second. Uranium, in addition to being radioactive, is readily corroded. Also, its soluble salts are quite toxic. However, uranium is not as likely as lead to accumulate in the body.
Because of its radioactivity, its tendency to corrode or other factors, uranium is usually accompanied by an overcoating of a non-radioactive, highly absorbent material, such as lead. In U.S. Pat. No. Re. 29,876, Reese discloses a depleted uranium container, with a corrosion-free coating of stainless steel, for transporting radioactive materials. Takeshima, in U.S. Pat. No. 5,015,863, uses depleted uranium particles coated with a metal of high thermal conductivity, such as aluminum, copper, silver, magnesium, or the like.
As for shielding neutrons, cadmium is the material most known for such use. Other neutron-absorbing materials exist, but do not absorb neutrons as well as cadmium and also have disadvantages that discourage their use. For example, hydrogen, the most common neutron absorber, is readily available and non-toxic, but hydrogen has a relatively small absorption cross-section, or probability of a nucleus absorbing a neutron. Also, lithium and boron, which are relatively better neutron absorbers, are both chemical poisons and are difficult to handle in the metallic state.
Cadmium-113 absorbs thermal (low energy) neutrons extremely well but, like uranium, is a radioactive material with a very long half-life. Also, cadmium is very toxic to humans, with effects on the central nervous system similar to those of mercury.
Because of the undesirable features of cadmium as a neutron absorber, gadolinium is sometimes substituted. Gadolinium is a rare-earth metal existing in seven natural isotopes. Only one of these isotopes is slightly radioactive, and it makes up only 0.2% of the total metal. Natural gadolinium averages only about one gadolinium-152 disintegration per gram in each ten minutes, and thus is considered to be non-radioactive for most purposes.
Gadolinium is used primarily in controlling the chain reaction in nuclear energy production. Gadolinium is also known as a shielding material, especially in storing radioactive materials, as is disclosed by Takeshima et al in U.S. Pat. No. 5,015,863 and Barnhart et al in U.S. Pat. No. 4,868,400.
Both gadolinium-155 and gadolinium-157 have much higher neutron absorption cross-sections than cadmium (three times and twelve times that of cadmium-113, respectively). Moreover, each of these isotopes makes up a higher percentage of gadolinium metal than does the isotope cadmium-113 in cadmium. Therefore, a neutron absorber made substantially of gadolinium does not have to be as pure as one made of cadmium to absorb thermal neutrons as effectively.
In nature, gadolinium occurs mixed with other rare-earth metals, but can be separated by well known techniques such as ion-exchange and the like. Gadolinium is malleable, ductile, and available in a number of forms, including sheets, foil and wire. Gadolinium is stable in dry air, but is attacked by acids and moist air. Thus, gadolinium requires varying degrees of protection for certain applications.
Despite the availability of radiation shield materials such as depleted uranium, which absorbs gamma rays, and gadolinium, which absorbs neutrons, there remains a need for more effective radiation shielding.